The invention relates to an apparatus as defined in the opening part of claim 1.
The invention also relates to a method as defined in the opening part of claim 7.
Such an apparatus is known from EP 569 597 A1. During reading of the data carrier a high-frequency signal and low-frequency signals are derived from the detection signal. From the high-frequency signal an information signal is derived and channel decoding and error correction can be effected. From the low-frequency signals further signals are derived for positioning the transducer relative to the data carrier and for focussing of a radiation beam produced by the transducer onto the data carrier. The data carrier has for example a radiation-sensitive layer, so that it can be inscribed and/or rewritten. The detection of the optical pattern on the data carrier is most reliable when the radiation beam is perpendicularly incident on the data carrier. As the density with which data is recorded on an optical data carrier increases the tolerance for deviations (tilt) of the angle of incidence from the perpendicular angle diminishes. The tilt can have a radial component and a tangential component. The tangential component (tangential tilt) is defined here as the component of the deviation in a plane oriented parallel to the track to be read and transversely to the data carrier. The radial component (radial tilt) is the component of the deviation in a plane oriented transversely to the track to be read and transversely to the data carrier.
In the known read/write apparatus the transducer comprises a transparent plate arranged in an optical path from a radiation source to the data carrier. The orientation of the transparent plate depends on an error signal which is a measure of the radial and/or tangential tilt of the data carrier. For this purpose, in a first embodiment, a sub-beam is split off the beam which is reflected from the data carrier. The sub-beam is imaged onto a four-quadrant detector via a screen which blocks a central portion of the beam. The error signal is derived from differences between four signals generated by means of the detector. In another embodiment the apparatus is equipped with auxiliary means for determining the orientation of the transparent plate and auxiliary means for determining the orientation of the data carrier. Both auxiliary means comprise a separate radiation source and a detector for generating four signals. In this embodiment the error signal is derived from the two sets of four signals.
It is an object of the invention to provide an apparatus of the type defined in the opening part, which requires less additional optical means for generating the error signal for the purpose of radial tilt correction.
To this end, according to the invention, the apparatus of the type defined in the opening part is characterized as defined in claim 1. To this end, the method of the type defined in the opening part is characterized as defined in claim 7.
In the apparatus in accordance with the invention the error signal is derived from the detection signal. Since this signal is generated anyway for the purpose of reading data from the data carrier no additional optical components are required.